Semiconductor device fabrication includes, for example, a process in which a pattern formed on a mask is transferred to a photosensitive substrate. During this process, if there is any defects and/or particles, e.g. dust, etc., adhering to the surface of the mask on which the pattern is formed, the image of the defects and/or particles will also be transferred to the substrate and cause circuit pattern defects. Therefore, before performing the pattern-transfer exposure, the pattern surface of the mask must be inspected for defects and/or particles, using a defects and/or particles optical detection system.
An example of an optical inspection system for detecting the presence of defects and/or particles on a pattern surface of a mask (reticle) is disclosed in Japanese Laid-Open patent H 5-87549. This optical system has a scanning optical system placed over the pattern side of the mask, and a light reception optical system placed over the same side.
A defect inspecting system using conventional technology is also disclosed in U.S. Pat. No. 4,468,120. This system employs a reception optical system arranged at a position in which diffracted light beams through the mask material will not enter.
The optical detection system of present invention employs a first light reception optical system placed on the pattern surface side of the mask, a second light reception optical system placed on the side of the mask opposite the pattern side and a corrective optical element for correcting differences in aberration states of the first and second optical systems. The light reception optical system of the present invention is arranged at a position at which it can receive both scattered light from defects on the mask and diffracted light from patterns on the mask. An alternative apparatus for detecting defects on a mask using both a reflection detection system and a transmission detection system is taught and described in Applicants corresponding copending U.S. Pat. application Ser. No. 08/636,511, entitled "Apparatus For Detecting Defects on A Mask", the disclosure of which is herein incorporated by reference.
The object of the present invention is to provide an optical detection system that is capable of detecting even small defects and/or particles on the surface of a mask with high sensitivity, unaffected by diffracted light from the edges of the patterns even when inspecting surface patterns on thick masks.